Download AgTM 331

AgTM 330
Week 14
Heating and Cooling
Objectives:
- List the units of measurement used in heating and cooling.
- List the three methods of heating spaces, animals and humans.
- Name at least four types of electric heating.
- Describe the types of thermostats used for heating and cooling equipment control.
- List the components of an oil or gas heater.
- Describe the installation of an electric forced-air heating system, which utilizes
Available heat in the building.
- Explain at least two farm uses of heating cable installed in concrete or asphalt floors
and slabs.
- Describe the installation of electric heating cable in floors and slabs.
- Size conductors and electrical components for an air conditioner.
- Explain the function of a heat pump.
Heating Units of Measurement
Temperature is the flow of heat as voltage difference between two wires
Resistance of material to the flow of heat is R-value
Heat is measured in several units
BTU British Thermal Units
kCal kilocalories
kWh kilowatt-hours
kJ
kilojoule
SI unit of heat
Electric Heaters are rated in kilowatts (kW)
ie, baseboard heaters (4 ft.=1000 watts, 6 ft = 1500 watts, 8ft.=2000 watts)
Fuel-fired furnaces are rated in Btu per hour (Btu/hr)
Heating and Cooling load calculation are generally made in (Btu/hr)
Air-conditioning units are rated in (Btu/hr) for smaller sizes and
For larger sizes in tons
Def. 1 ton of refrigeration is the amount of heat absorbed by 1-ton block of ice
in 24-hour period.
Stopped on 4-20-11
Therefore heating is rated in heat per unit of time.
Trend toward
Kilowatt
Kilojoule
Megajoule
(kW)
(kJ) joule is a small unit of measure of heat
(MJ) 1000 kJ
Conversion factor for heating and cooling units.
Multiply By
To Find
BTU
1.055 Kj
KCal
4.187 Kj
KWh
3600 Kj
KWh
3.6
Mj
KW
3413 Btu/hr
Tons
12000 Btu/hr
Tons
3.516 KW
KW
3600 Kj/hr
Btu/hr
1.055 Kj/hr
KCal/hr 4.187 Kj/hr
Principles of Transferring Heat
1. Convection
 Moving heat through a fluid such as air
 Main method of moving heat for most heating and cooling systems
(forced air throughout the space to be heated)
 Natural convection, baseboard heaters.
 Convection ovens
2. Conduction
 moving of heat from one object to another object in touches
 heat moves through walls, floors, windows and ceilings
 Animals cool them selves by conduction by lying down on the ground
 Heating cables in the floor for animals
3. Radiation
Process of emission, transmission and absorption of radiant energy
Heat is emitted from an object and travels in the form of electromagnetic radiation
heat can be moved directly from the source to a person, animal or object
Heat from the sun reaches earth by radiation through emptiness of space
Heating with electricity
Heating is accomplished by passing current through a wire containing a desired
amount of resistance. Baseboard, toasters, hairdryers, curling irons
Infrared heat lamps and quartz infrared tubes use tungsten filaments.
Electric resistance heaters use nichrome wire.
Example;
A 2-kW electric resistance heater designed to operate at 235 V has a resistance of
27.7 ohms and produces 2 kWh of heat in 1 hour.
Equivalent to 6826 Btu/ht or 7.2 MJ
Using equation 2.7 where Heat = (I2) ( R)(t)
Remember heat is in Wh.
Types of Electric Heat
1. Baseboard heater
resistance style, good for clean areas, isolated
ie. Notice items above the heater
2. Self-contained heater
wall or ceiling mounting with fan to force air over heating elements
3. Electric furnace
Self-contained unit with resistance heating elements and a blower
force the warm air through a duct system
good where they can extract heat from some heat source.
Milk, cooling towers, methane
4. Infrared heater
Temporary heat source for newborn animals, work areas
5. Ceiling cable
radiant electric heat where resistance cable is embedded in ceiling plaster
which heats objects in the room
6. Floor cable
buried in floor and steps
common in vegetable grading, packing sheds, logging shops, animal
passageways, and baby piglets.
7. Thermostats
Switch operated by temperature change
Two types
Heating or cooling
Heating type closes, a switch that turns a heater on when temperature falls and opens
when opens when the temperature rises.
Cooling thermostat closes when the temperature rises and turns on an air conditioner,
refrigeration unit or ventilation fan and then opens when the temperature falls
Styles
 Low voltage 24 v
 Line voltage i.e. Baseboard
8. Sizing Circuit wires and over current protection
branch-circuit wire supplying an individual electric heating device must have a
rating not less than 1.25% times the ampere rating of the heater, plus any motors
Example, single-phase electric unit heat rated at 7.5 kW, 240 v and has fan motor that
draws 1 amp at 240 V.
heater draws 31.3 amps plus 1 amp for a total of 32.3 amp
(32.3amp)(125%) = 40Amp breaker
Minimum wire size is found in NEC Table 310-16
Copper wire the minimum size is no. 8 AWG THW
Example: A farm office is heated with two baseboard heaters on the same circuit.
Heater 1 is rated at 8.3 amp 2000W
Heater 2 is rated at 6.3 amps, 1500 W
Both are rated for 240- Voltage
Total of heaters is 14.6 A, therefore based on wire size of 125% of this value = 18.3 amp
Minimum size wire is #1-12 AWG copper and protected with a 20-amp breaker.
Most line voltage thermostats are rated at no more than 22 amps and many have lower
Ratings, check the box prior to installation.
Def. Heating contactor is an electrically operated switch.
High-wattage heaters are usually supplied with a contactor.
Line voltage thermostats are directly tapped into the power side of the line to heater.
Only voltage would be possible be the coil with only 1 amp.
Drawback, is of shorted the line thermostat will remain on.
This is called a Class 1, remote controlled – result is a fire hazard.
Remote-controlled line voltage thermostat must be in EMT, IMC, or rigid
Nonmetallic.
Problem 15-1
A single-phase electric unit heater is rated at 7.5 kW at 235 V, and has a full-lead
current rating of 32 Amp. The heater is controlled with a contactor operated with a linevoltage thermostat. The heater circuit wire is # 8 AWG copper THW, and the overcurrent device is rated at 40Amp. Determine the minimum size TW copper wire
permitted for the thermostat if there are no fuses in the heater to protect the thermostat
wire.
Solution.
The heater circuit over-current device is not permitted to exceed three times the
thermostat wire ampere rating. a # 14 AWG TW copper wire has a rating of 20 Amp.
No. 14 AWG copper TW wire is permitted for the thermostat.
Problem 15-2
Two 3200W, 240 Volt quartz infrared heaters are installed in a barn and supplied
by one circuit. The heaters are controlled with a contactor operated by a line-voltage
thermostat. Determine the following.
 Minimum heater circuit wire size.
 Minimum size circuit breaker for the heater circuit
 Minimum size remote-control thermostat wire size not requiring separate over
current protection.
All of the wire is THWN copper in conduit
Solution.
The ampere rating of each infrared heater was not given in the problem, but it can
be calculated easily.
Current = 3200 Watt/240volt = 13.3 amp, plus there are two heaters in the circuit. For a
total of 26.6 amp
Minimum branch wire = 1.25 x 26.6 amp = 33.3 amp
NEC Table 310-16, the minimum size wire is no. 8 AWG copper THWN
Sizing the circuit breaker for 33.3 amp, use a 40-amp breaker
Thermostat wire ampere rating = 40 amp/3 = 13.3 amp so
THWN wire with an ampere rating of at least 13.3 amp = No. 14 AWG is
adequate.
Disconnect
Electric heater required to have a disconnect that will open any ungrounded wire
to the heater.
Must be with in sight of the heater unit
Used on electric heating equipment of less than 1/8 hp
A switch on the electric heating equipment with a marked off position that opens
all ungrounded wire may serve as the disconnect
Circuit breaker or fusible switch protecting the heater circuit does not need to be
insight from the heater, but it must be accessible.
A line-voltage thermostat may be temperature operated only, or it may also have a
manual off position. Where the thermostat is turned to the off position, it cannot be
turned on by a change in temperature.
A thermostat with a manual off position is permitted to serve as a disconnect for
electric heating equipment, provided it opens all ungrounded (hot) wires
Single – pole thermostat, can serve if off position and only 120 voltage
Double-pole thermostat is required for 240 voltage
Single –pole thermostat for 240-voltage does not serve as a disconnect.
High-Temperature Limit Switch
Space heating contains high-temperature limit switch
Furnace blowers that opens the circuit and shuts off the heat before danger of fire
Switches are wired in series with the electric heating element.
If the high-temperature switch fails and becomes permanently open, heater will not work
and can be tested with a continuity tester.
Installing Electric Heating Equipment
 Protected from physical abuse
 Resistant to corrosion and moisture
 Contactors installed in damp areas should be periodically treated with suitable
Lubricant to prevent rust and operating freely
 Kept a safe distance from combustible materials
 Grounding is extremely important in ag application
Fuel-Fired Heaters
Space heaters fired with gas or oil.
Air handling systems, ducts, and ventilation systems
Water radiators, baseboard units, which requires an electric circulating pump.
Divided into sub-zones
Usually runs with a low-voltage thermostat
Disconnect in sight of the furnace
Time-delay fuses sized at 1.15 times the motor nameplate current
Electric Heating Cable in Floors and Slabs
Ready to install mats either 10W for supplemental heating in northern climates.
20W per square foot are usually adequate to provide total heating for most inside
areas
Heat mats for swine are as high as 40Watts per square foot
Can go as high as 60 Watts per square foot
Heat Sources in Agricultural Buildings
Animal, product and natural heat produce significant quantities to augment a
space heating systems
Mature dairy cow can produce up to 3000 Btu of heat per hour
People at normal work produce 1000 Btu per hour.
Fruit and vegetables products produce heat while in storage
Heat of respiration, which must be removed by refrigeration system
Natural heat – sun
Heating cable may not be needed if building orientation or color
Example of room remodel in bedroom
Darrell’s water heater
Window arrangement winter vs. summer
Heat from lights and machinery contribute to heating system
Definition of plenum is an enclosed space in which various sources of natural and
mechanical heat are utilized together to augment the heating system
Animals and humans give off moisture into a room as well as heat
Ventilation is required to eliminate excess moisture
Orange peel in room and roofs
Pipe Heating Cables
Usually plug-in type
Wrapped around the pipe in a spiral pattern and never should cross itself or
overheating will result
Heat tapes for pipelines are available with thermostat control located close to the
desired heating activities.
Energy Guide
Air conditioners and household refrigeration appliances have an energy guide that gives
an estimated cost
Air conditioners contains an energy efficiency rating (EER)
Refrigeration Principles
Refrigeration removes heat from a room or object, because ice absorbs 144 BTU of heat
per hour for every pound of ice that melts. The heat absorbed to simply melt the ice is
called latent heat.
Refrigeration system compressor simply pumps the refrigerant throughout the system.
The refrigerant repeatedly changes back and forth from a liquid to gas so the liquid
evaporates, it absorbs heat and when it condenses, it gives off heat.
The compressor pumps refrigerant vapor into the condenser where it changes to a liquid
and gives off heat.
The liquid then travels to the expansion valve.
The pressure suddenly drops at the expansion valve, and the liquid begins to evaporate.
The liquid refrigerant is completely evaporated by the time it leaves the evaporator.
Heat is absorbed by the refrigerant in the evaporator.
The refrigerant vapor returns to the compressor where it begins anther cycle.
The refrigerant absorbs heat at the evaporator and gets rid of heat at the condenser.
Common Refrigerants
Today, there are three specific types of refrigerants used in refrigeration and airconditioning systems:
1. Chlorofluorocarbons or CFCs, such as R-11, R-12, and R-114
2. Hydrochlorofluorocarbons or HCFCs, such as R-22 or R-123
3. Hydrofluorocarbons or HFCs, such as R-134a. All these refrigerants are
"halogenated," which means they contain chlorine, fluorine, bromine, astatine, or
iodine.
Refrigerants, such as Dichlorodifluoromethane (R-12), Monochlorodifluoromethane (R22), and Refrigerant 502 (R-502), are called PRIMARY REFRIGERANTS because each
one changes its state upon the application or absorption of heat, and, in this act of change,
absorbs and extracts heat from the area or substance.
The primary refrigerant is so termed because it acts directly upon the area or
substance, although it may be enclosed within a system. For a primary refrigerant to
cool, it must be placed in a closed system in which it can be controlled by the pressure
imposed upon it. The refrigerant can then absorb at the temperature ranges desired. If a
primary refrigerant were used without being controlled, it would absorb heat from most
perishables and freeze them solid.
SECONDARY REFRIGERANTS are substances, such as air, water, or brine. Though
hot refrigerants in themselves, they have been cooled by the primary refrigeration system;
they pass over and around the areas and substances to be cooled; and they are returned
with their heat load to the primary refrigeration system. Secondary refrigerants pay off
where the cooling effect must be moved over a long distance and gastight lines cost too
much.
Refrigerants are classified into groups. The National Refrigeration Safety Code catalogs
all refrigerants into three groups:



Group I – safest of the refrigerants, such as R-12, R-22, and R-502
Group II – toxic and somewhat flammable, such as R-40 (Methyl chloride) and R764 (Sulfur dioxide)
Group III – flammable refrigerants, such as R-170 (Ethane) and R-290 (Propane).
R-12 DICHLORODIFLUOROMETHANE (CC12 F2 ) Dichlorodifluoromethane,
commonly referred to as R-12, is colorless and odorless in concentrations of less than 20
percent by volume in air. In higher concentrations, its odor resembles that of carbon
tetrachloride. It is nontoxic, noncorrosive, nonflammable, and has a boiling point of 21.7°F (-29°C) at atmospheric pressure.
WARNING
Because of its low-boiling point at atmospheric pressure, it prevents liquid R12 from
contacting the eyes because of the possibility of freezing.
One hazard of R-12 as a refrigerant is the health risk should leakage of the vapor come
into contact with an open flame of high temperature (about 1022°F) and be decomposed
into phosgene gas, which is highly toxic. R-12 has a relatively low latent heat value, and,
in smaller refrigerating machines, this is an advantage.
R-12 is a stable compound capable of undergoing the physical changes without
decomposition to which it is 6-20.commonly subjected in service.
The cylinder code color for R-12 is white.
R-22 MONOCHLORODIFLUOROME-THANE (CHCIF2 )
Monochlorodifluoromethane, normally called R-22, is a synthetic refrigerant developed
for refrigeration systems that need a low-evaporating temperature, which explains its
extensive use in household refrigerators and window air conditioners. R-22 is nontoxic,
noncorrosive, nonflammable, and has a boiling point of -41°F at atmospheric pressure.
R-22 can be used with reciprocating or centrifugal compressors. Water mixes readily
with R-22, so larger amounts of desiccant are needed in the filter-driers to dry the
refrigerant.
The cylinder code color for R-22 is green.
R-502 REFRIGERANT (CHCIF2 /CCIF2 CF3 ) R-502 is an azeotropic mixture of
48.8 percent R-22 and 51.2 percent R-115. Azeotropic refrigerants are liquid mixtures of
refrigerants that exhibit a constant maximum and minimum boiling point. These mixtures
act as a single refrigerant. R-502 is noncorrosive, nonflammable, practically nontoxic,
and has a boiling point of -50°F at atmospheric pressure. This refrigerant can only be
used with reciprocating compressors. It is most often used in refrigeration applications
for commercial frozen food equipment, such as frozen food walk-in refrigerators, frozen
food display cases, and frozen food processing plants.
The cylinder color code for R-502 is orchid.
R-134a TETRAFLUOROETHANE (CH2 FCF3 ) R-134a, tetrafluoroethane, is very
similar to R-12, the major difference is that R-134a has no harmful influence on the
ozone layer of the earth's atmosphere and is a replacement for R-12 applications.
Noncorrosive, nonflammable, and nontoxic, it has a boiling point of -15°F at atmospheric
pressure. Used for medium-temperature applications, such as air conditioning and
commercial refrigeration, this refrigerant is now used in automobile air-conditioners.
The cylinder color code for R-134a is light (sky) blue.
Additional Refrigerants
In addition to the previously mentioned refrigerants, other less common refrigerants are
used in a variety of applications.
R-717 Ammonia (NH3 ) Ammonia, R-717, is commonly used in industrial systems. It
has a boiling point of -28°F at atmospheric pressure. This property makes it possible to
have refrigeration at temperatures considerably below zero without using pressure below
atmospheric in the evaporator. Normally it is a colorless gas, is slightly flammable, and,
with proper portions of air, it can form an explosive mixture, but accidents are rare.
The cylinder color code for R-717 is silver.
R-125 Pentafluoroethane (CHCF5 ) Pentafluoroethane, R-125, is a blend component
used in low- and medium-temperature applications. With a boiling point of -55.3°F at
atmospheric pressure, R-125 is nontoxic, nonflammable, and noncorrosive. R-125 is one
replacement refrigerant for R-502.
All refrigerants have their own characteristics. It is extremely important to charge a
system with the refrigerant specified. Use of an incorrect refrigerant can lead to reduced
efficiency, mechanical problems, and dangerous conditions.
Vapor-compression cycle
The vapor-compression cycle is used in most household refrigerators as well as in many
large commercial and industrial refrigeration systems. Figure 1 provides a schematic
diagram of the components of a typical vapor-compression refrigeration system.
Figure 1: Vapor compression refrigeration
The thermodynamics of the cycle can be analyzed on a diagram as shown in Figure 2. In
this cycle, a circulating refrigerant such as Freon enters the compressor as a vapor. From
point 1 to point 2, the vapor is compressed at constant entropy and exits the compressor
as a vapor at a higher temperature, but still below the vapor pressure at that temperature.
From point 2 to point 3 and on to point 4, the vapor travels through the condenser which
cools the vapor until it starts condensing, and then condenses the vapor into a liquid by
removing additional heat at constant pressure and temperature. Between points 4 and 5,
the liquid refrigerant goes through the expansion valve (also called a throttle valve)
where its pressure abruptly decreases, causing flash evaporation and auto-refrigeration of,
typically, less than half of the liquid.
Figure 2: Temperature–Entropy diagram
That results in a mixture of liquid and vapor at a lower temperature and pressure as
shown at point 5. The cold liquid-vapor mixture then travels through the evaporator coil
or tubes and is completely vaporized by cooling the warm air (from the space being
refrigerated) being blown by a fan across the evaporator coil or tubes. The resulting
refrigerant vapor returns to the compressor inlet at point 1 to complete the
thermodynamic cycle.
The above discussion is based on the ideal vapor-compression refrigeration cycle, and
does not take into account real-world effects like frictional pressure drop in the system,
slight thermodynamic irreversibility during the compression of the refrigerant vapor, or
non-ideal gas behavior (if any).
More information about the design and performance of vapor-compression refrigeration
systems is available in the classic Perry's Chemical Engineers' Handbook.[13
Hermetic Compressor
A completely sealed system with a motor and compressor
Refrigeration Controls
Thermostats and pressure switches are generally used to control the system
Nameplate is the key for the system
Disconnect
A disconnect must be located within sight for the motor compressor where accessible
Sizing the system is usually similar to other calculation.
Multiply the amperage by 1.15 to the branch-circuit amperage.
Motor Overload Protection
Compressor motor must be protected from overload with a thermally protected
compressor motor or a time delay fuse sized at not more than 125% of the full-load
current.
Room Air Conditioners
Heat Pump
Is a space heating system and a central air conditioner in one unit
Actually a refrigeration system
Heat pumps are of two types
1. Air to air
2. Water to air
1. This has an outside coil which takes heat from the air and by means of refrigerant
discharges the heat into the inside air duct.
2. This system uses sources of water are wells, rivers and lakes
In the winter the heat pump removes heat from the water and delivers it to the
inside heating ducts.
In the summer, heat is removed from the air flowing though the ducts and
delivered to the water flowing through the other coil
In northern climates a heat pump is augmented with resistance heating elements in the
ductwork and the resistance heaters turn on the supply the additional heat necessary